A slide fastener generally comprises a pair of stringer tapes along confronting edges of which are mounted rows of coupling heads or teeth which interdigitate upon movement of a slider along these rows. The tapes and the rows of coupling heads form stringers and the slide fastener may be provided at its ends with stop members preventing withdrawal of a slider, means for coupling the two tapes or slide-fastener halves together in the case of a nonseparable slide fastener, or male and female formations enabling the coupling and decoupling of the stringer halves in the case of a separable slide fastener.
The tapes generally are incorporated, e.g. by stitching, in opposite sides of an article having an opening which can be closed by the slide fastener.
The slide-fastener art has advanced rapidly in recent years both with respect to techniques for the fabrication of the stringers and with respect to improvements in the mechanical and esthetic properties thereof. For example, woven support tapes which have hitherto been provided with the coupling heads by clamping, by molding or by rows of stitching, have increasingly been provided with the heads directly during the tape-weaving operation utilizing conventional tape looms which can be modified to allow the coupling heads to be mounted on the edges of the tape as the tape is woven.
In recent years, moreover, individually mounted coupling heads have tended to be replaced by continuous coupling elements composed of a synthetic resin monofilament and of coil or meander configuration. The individual coupling heads can be formed by deforming, e.g. molding, bending or kinking the monofilament at spaced-apart locations, to so shape the head that it has lateral protrusions which can engage behind the heads of the opposing coupling element when the two are interdigitated, i.e. when the heads of one coupling element are fitted into the interstices or interhead spaces of the opposing coupling element.
The heads have shanks which reach toward the respective tapes and can be connected to the shanks of adjoining heads by connecting members or bights. Each head and the associated pair of shanks will be referred to hereinafter as a coupling member and because coils of the type described have a generally dentate configuration, the coupling heads or coupling members may be referred to as "teeth". In referring to woven support tapes, reference will be made to the ground weave. This, of course, is intended to mean the woven fabric consisting of weft yarns and warp yarns, to which the coupling element is secured. A ground weft is, therefore, a weft yarn structure of the ground weave. A "double weft" is a weft of a single yarn having two passes received in the same warp pockets or shed in traversing the tape.
It is known, e.g. from German open application (Offenlengungsschrift) DE-OS No. 2 219 907 and the corresponding U.S. Pat. No. 3,880,203 issued Apr. 29, 1975 to Alfons Frohlich, one of the present joint inventors, to provide stringer tapes which are composed of warp yarns and double wefts formed from a continuous ground weft yarn with the coupling element being woven into the tape or secured to the tape by weaving. In this case, the two shanks of each coupling member can also form a double weft which is secured to the ground weave by warp yarns at least in part.
The teeth or coupling members have their shanks superposed, i.e. an upper shank lies directly over a lower shank so that the two shanks of each coupling member and connected to a respective coupling head, have a common projection in the slide fastener plane. In this construction, moreover, double wefts of the ground weft yarns are disposed between the coupling members. These double ground wefts can be received in their own warp yarn pockets, i.e. the pockets in the warp resulting from shedding the warp during weaving and defined as the pockets between consecutive crossings of the warp in warp patterning.
While the stringer of these publications has been found to be excellent for most purposes, it is susceptible to damage by a sharp transverse pull upon the coupling elements toward the coupling head side and away from the tape. Furthermore, the afore-mentioned weaving attachment does not always provide satisfactory flexibility of the fastener.
It has been found also that longitudinal tension or stress upon the stringer tape, or shrinkage of the latter, has a tendency to vary the spacing or pitch between the teeth because the spacing between two shanks of different coupling members received in a common warp pocket responds to deformation thereof.
Finally, while the tapes with the coupling elements woven into place can be fabricated on conventional double-pick needle tape looms, the particular system of securing the coupling elements has been found to limit the speed at which the loom can operate.
Slide fasteners which are improvements over those of the above-mentioned publication are described in our copending application Ser. No. 195,000, filed Oct. 9, 1980 in which each connecting warp yarn is offset from the next by two weft picks, each such yarn engaging alternately in a repeating pattern above a pair of coupling members and above the next coupling member, then below the subsequent coupling member. Between the pair of coupling members and the following coupling member overshot by the connecting warp yarn, the lock engages below an interstitial ground weft yarn forming a double weft disposed between the pair of coupling members and the next coupling member.
The connecting warp yarns also can engage over the double weft of the ground weft yarn disposed between the individual overshot coupling member and the next undershot coupling member.
In a slide fastener of this type the double warps of the ground weft yarn and the monofilament double weft are disposed in separate warp pockets disposed one above another transverse to the fastener plane.
The result is a particularly soft and flexible slide fastener since the weft joints of the stringer tapes and the stringer tape as a whole can move above the rows of coupling members in response to bending stresses in the fastener plane and in response to kinking forces applied to the slide fastener. Another advantage is that the pitch (interhead spacing) of the slide fastener remains very stable and does not materially alter as subjected to dyeing, washing or ironing.
However, slide fastener of the type described in our copending application do not always fully satisfy all requirement with regard to antikinking properties. The term "kinking" is here used to refer to a distortion of the slide fastener in response to sharp bends up or down of the stringer from the fastener plane. The antikinking property of kink resistance is a measure of the transverse tensile strength in response to the up or down bending movements from the fastener plane. This property increases substantially parabolically with radius of curvature near the kinking point or vertex, i.e. the smaller the kinking angle, the worse is the ability of the stringer to withstand the kinking.